The present invention relates to a vertical drainage device adapted to be embedded in soil such as ground or banking for sucking up water in the soil and draining the soil. More particularly, the present invention relates to a vertical drainage device suitable for improving the ground which can drain the soil such as a weak ground containing a large amount of water up to the surface of the ground.
In recent years, various soil improving methods have been carried out wherein many drainage members each having a non-woven fabric are vertically embedded in the weak ground, so that the water in the ground may be raised through the drainage members up to the surface of the ground. FIG. 12 shows an example of the soil improving methods. Referring to FIG. 12, many drainage members 200 each having a non-woven fabric are vertically embedded in a weak ground 201 so as to be arranged at equal intervals. A drainage layer (e.g., sand mat) 202 is laid on the surface of the weak ground 201. A pore water existing in the weak ground 201 is removed through the drainage members 200 and the drainage layer 202 up to the surface of the weak ground 201, and is then discharged to the outside by a drainage pump 203. A banking 204 is laid on the drainage layer 202, so as to apply a load to the weak ground 201, thereby increasing a water pressure to promote the drainage.
Various forms of such a drainage device are known. For example, the drainage device is constituted of a cylindrical non-woven fabric and an elongated plate-like synthetic resin conduit member disposed in the cylindrical non-woven fabric. Some examples of the synthetic resin conduit member are shown in FIGS. 13 and 14. The conduit member shown in FIG. 13 (which will be hereinafter referred to as B type) is constructed as a thin plate formed with many parallel ribs extending in a longitudinal direction of the plate on opposite sides thereof. On the other hand, the conduit member shown in FIG. 14 (which will be hereinafter referred to as C type) is constructed as a corrugated plate formed of a synthetic resin.
While it is an important subject in designing of such a drainage device to improve a drainage efficiency, the above-mentioned conventional drainage device is not yet satisfactory in the drainage efficiency.
Another conventional drainage device intended to improve the drainage efficiency is disclosed in Japanese Patent Application No. 63-11838 (Japanese Patent Laid-open Publication No. 63-315722) based on British Patent Application Nos. 8701259, 8707545 and 8719584.
The drainage device disclosed in this cited reference is shown in FIGS. 15, 16 and 17. This drainage device comprises a mesh structure formed by a plurality of substantially parallel main strands 12 and a plurality of substantially parallel auxiliary strands 13 connecting the main strands 12 together, and a water permeable member 16 surrounding the mesh structure, wherein an outer surface of the auxiliary strands 13 is flush with one surface of the mesh structure, whereby a main flow passage is defined between the adjacent ones of the main strands 12 so as to extend in parallel to a longitudinal direction of the main strands 12, and an auxiliary flow passage is additionally defined between the adjacent ones of the auxiliary strands 13 so as to extend in parallel to a longitudinal direction of the auxiliary strands 13, and wherein each main strand 12 has a height at least twice a height of each auxiliary strand 13, so that a ratio of a free cross sectional area of each main strand 12 to a free cross sectional area of each auxiliary strand 13 is set to at least 2.5:1.
As shown in FIG. 17, the drainage device is vertically embedded in the soil, and it is connected at its lower end to a conduit 19 which is in turn connected to a sewer 20, thus constructing a drainage system. The water existing in the soil is caught by the drainage device, and is then allowed to fall through the main flow passages each defined between the adjacent main strands 12. Then, the water flows into the conduit 19 to be gathered in the sewer 20. Finally, the water is discharged from the sewer 20 to the outside.
As mentioned above, the drainage device disclosed in Japanese Patent Laid-open Publication No. 63-315722 is constructed so as to allow the water in the soil to fall through the main flow passages defined by the main strands. To increase a water flow in the main flow passages, the height of each main strand is set to at least twice the height of each auxiliary strand, so that the ratio of the free cross sectional area of each main strand to the free cross sectional area of each auxiliary strand is set to at least 2.5:1. Furthermore, a ratio of a spacing of the adjacent main strands to a spacing of the adjacent auxiliary strands is set to from 1.5:1 to 5:1, preferably 2:1. The spacing of the adjacent main strands is typically 8 mm according to the description in this cited reference.
However, the drainage device disclosed in Japanese Patent Laid-open Publication No. 63-315722 is intended to lower the water in the soil through the main flow passages defined by the main strands rather than to raise the water in the soil as by the drainage device of the present invention. Accordingly, although it is considered that the water flow may be increased by setting the spacing between the adjacent main strands to 8 mm in the case of lowering the water through the main flow passages, it has been made apparent from various tests that such spacing of 8 mm is too wide in the case of raising the water in the soil.